1. Field of the Invention
The invention concerns a process for manufacturing precisely fitting dental castings.
2. Description of the Related Art
In the manufacture of dental castings, such as crowns, bridges and other dental prosthesis, after preparation of the tooth stumps, the dentist produces an impression of the tooth stumps. This impression is given to a dental laboratory, where the impression is filled in with a modeling material, generally plaster. The resulting model is used by the dental technician upon which to model and cast the cast objects (castings). For this the castings are modeled out of wax on the model of the tooth stumps. The wax models are embedded in a matrix, so that after melting out of the wax a hollow shape for the pouring of the castings is formed. This process is described, for example, in DE 94 12 336 U1.
If the castings made in this manner are then seated upon the tooth stumps of the patient, fitting imperfections are frequently discovered. These fitting imperfections can have numerous causes. For example, many imprint materials are available in the market with varying chemical compositions, such as for example silicones, polyether rubbers and hydrocolloids. These various materials exhibit various shape changes, usually shrinkage, during hardening or setting. Also, the utilization of varying hardeners and varying mixture relationships influence the physical properties of the imprint material. Further, the dimensions of the cast object are influenced by the various plaster model materials used for filling in the impression or negative. Thereby variations of between approximately 0.2 and 0.06 vol.% are produced in the setting or hardening expansion of the model material. Also, the mixing relationship of water to plaster powder, and the selection of distilled water or tap water with mineral contents, influences the setting or hardening relationship in the manufacture or production of the model. The largest influence on the dimension of the cast object is the mold matrix. The conventionally available mold matrix materials exhibit an expansion of between approximately 0.5 and 3.5 vol.%, wherein these expansions can be controlled or influenced by the mixing relationship and the utilization of distilled water or special mixing fluids.
In the hitherto known procedures the dental technician attempts by exercising control over the embedding matrix to produce cast objects which have an optimal form fitting precision with respect to the plaster model produced by him. The form fitting precision to the plaster model for the above discussed reasons does not however necessarily mean a corresponding form fitting precision to the tooth stumps of the patient. Thus, as a rule, there occurs a further follow-up working of the casting by the dentist or as the case may be the dental laboratory. This is associated with a high labor expenditure for the dentist and the laboratory. The multiple follow-up grindings and fittings of the casting is also inconvenient and time consuming for the patient. Finally, the follow-up working of the casting does not always lead to an optimal occlusion, which must be compensated for by larger cement fisures, which increase the vulnerability to secondary cavities.